Joined body of piezoelectric material substrate and support substrate, and acoustic wave element

ABSTRACT

A bonded body includes a supporting substrate, a piezoelectric material substrate of a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate, and a bonding layer bonding the supporting substrate and piezoelectric material substrate. The material of the bonding layer is silicon oxide. Provided that the bonding layer is divided into a piezoelectric material substrate-side bonding part and supporting substrate-side bonding part, the piezoelectric material substrate-side bonding part has a nitrogen concentration higher than a nitrogen concentration of the supporting substrate-side bonding part.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of PCT/JP2018/042433, filed Nov. 16, 2018, which claims priority to Japanese Application No. 2017-252872, filed Dec. 28, 2017, the entire contents all of which are incorporated hereby by reference.

TECHNICAL FIELD

The present invention relates to a bonded body of a piezoelectric material substrate and supporting substrate, and an acoustic wave device.

BACKGROUND ARTS

It has been widely used an SOI substrate composed of a high resistance Si/SiO₂ thin film/Si thin film, for realizing a high-performance semiconductor device. Plasma activation is used for realizing the SOI substrate. This is because the bonding can be realized at a relatively low temperature (400° C.). It is proposed a composite substrate composed of similar Si/SiO₂ thin film/piezoelectric thin film for improving the characteristics of a piezoelectric device (patent document 1). According to patent document 1, the piezoelectric material substrate composed of lithium niobate or lithium tantalate and silicon substrate with a silicon oxide layer formed thereon are activated by ion activation method, followed by the bonding.

It is further proposed a filter of multi-layered structure by forming a single or multiple dielectric film at an interface (patent document 2). However, there is few known information relating to bonding techniques for realizing the structure of lithium tantalate/silicon oxide/silicon.

Patent document 3 discloses that lithium tantalate and sapphire or ceramic are bonded through a silicon oxide layer by plasma activation method.

According to non-patent document 1, it is described that lithium tantalate substrate and a silicon substrate with a silicon oxide film provided thereon are bonded with each other by irradiating O₂ RIE plasma (13.56 MHz) and microwave (2.45 GHz) of N₂ in series.

When Si and SiO₂/Si are bonded with each other by plasma activation, a sufficiently high bonding strength is obtained by the formation of Si—O—Si bond at the interface. Further, at the same time, Si is oxidized to SiO₂ so that the flatness is improved and the bonding as described above is facilitated at the uppermost surface (Non-patent document 2).

Further, according to a surface acoustic wave device described in patent document 4, it is proposed that a film of a high sound speed and a film of a low sound speed are provided between a piezoelectric material substrate and supporting substrate to prevent the leakage of an energy of a surface acoustic wave into a supporting substrate.

RELATED TECHNICAL DOCUMENTS Non-Patent Documents

(Non-Patent Document 1)

-   ECS Transactions, 3 (6) 91-98 (2006)

(Non-Patent Document 2)

-   J. Applied Physics 113, 094905 (2013)

PATENT DOCUMENTS

-   (Patent document 1) Japanese Patent Publication No. 2016-225537A -   (Patent document 2) Japanese Patent No. 5910763B -   (Patent document 3) Japanese Patent No. 3774782B -   (Patent document 4) Japanese Patent No 5713025B

SUMMARY OF THE INVENTION

As described in the prior documents, in the case that as piezoelectric device is produced by thinning a lithium niobate or lithium tantalate substrate by ion injection, the characteristics is low which is problematic. It is considered that the crystallinity is deteriorated due to the damage during the ion injection.

On the other hand, in the case that a piezoelectric material substrate such as lithium niobate or lithium tantalate is bonded to a silicon oxide layer on a silicon substrate and that the piezoelectric material substrate is then polished to make the substrate thinner, a processing denatured layer can be removed by CMP so that the device characteristics are not deteriorated. However, as the thickness of the piezoelectric material substrate is made smaller by polishing, the characteristics of the thus obtained bonded body may be deteriorated. Particularly in the case that the bonded body is used as an acoustic wave device, it is proved that the characteristics as the acoustic wave device, particularly a ratio (admittance ratio) of an admittance at a resonance frequency fs with respect to an admittance at an anti-resonance frequency fr, is reduced.

An object of the present invention is, in bonding a piezoelectric material substrate of a material, selected from the group consisting of lithium niobate, lithium tantalate and lithium-niobate-lithium tantalate, and a supporting substrate with a silicon oxide layer, to improve the performance of a bonded body.

Solution for the Object

The present invention provides a bonded body comprising:

a supporting substrate;

a piezoelectric material substrate comprising a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate; and

a bonding layer for bonding the supporting substrate and the piezoelectric material substrate,

wherein the bonding layer comprises a material of silicon oxide; and

wherein a nitrogen concentration of a piezoelectric material substrate-side bonding part is higher than a nitrogen concentration of a supporting substrate-side bonding part, provided that the bonding layer is divided into the piezoelectric material substrate-side bonding part and the supporting substrate-side bonding part.

The present invention further provides a bonded body comprising:

a supporting substrate;

a piezoelectric material substrate comprising a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate; and

a bonding layer for bonding the supporting substrate and the piezoelectric material substrate,

wherein the bonding layer comprises a material of silicon oxide; and

wherein a nitrogen concentration of an intermediate part is higher than a nitrogen concentration of a piezoelectric material substrate-side bonding part and a nitrogen concentration of a supporting substrate-side bonding part, provided that the bonding layer is divided into the piezoelectric material substrate-side bonding part, the supporting substrate-side bonding part and the intermediate part between the piezoelectric material substrate-side bonding part and the supporting substrate-side bonding part.

The present invention further provides an acoustic wave device comprising

the bonded body and

an electrode provided on the piezoelectric material substrate.

Effects of the Invention

The inventors have researched the reason that the characteristic of the bonded body is deteriorated when the piezoelectric material substrate composed of lithium niobate or the like and the supporting substrate with the silicon oxide layer are directly bonded with each other. Particularly, it is studied in detail the reason that a ratio (admittance ratio) of an admittance at a resonance frequency fs with respect to an admittance at an anti-resonance frequency fr is reduced.

In the case that the bonding layer between the piezoelectric material substrate and supporting substrate is made of silicon oxide, a sound speed of bulk wave propagating in the silicon oxide (bonding layer) is lower than a sound speed of bulk wave propagating in the piezoelectric material substrate (for example lithium niobate or lithium tantalate). It is thus considered that propagating energy is leaked from the piezoelectric material substrate into the supporting substrate through the bonding layer to deteriorate the propagation characteristics.

Here, the inventors reached the idea of providing a part having a relatively high nitrogen concentration and having a relatively high sound speed on the side of the piezoelectric material substrate of the bonding layer and of providing a part having a relatively low nitrogen concentration and low sound speed on the side of the supporting substrate. As a result, it is found that the leakage of an acoustic wave propagating in the piezoelectric material substrate into the supporting substrate can be suppressed.

Further, the inventors reached the idea of providing a part having a relatively high nitrogen concentration and a relatively high sound speed in the intermediate part of the bonding layer and of providing parts each having a relatively low nitrogen concentration and low sound speed on the sides of the supporting substrate and the piezoelectric material substrate. It is thus found that the leakage of the acoustic wave propagating in the piezoelectric material substrate into the supporting substrate is suppressed to provide a device of a low loss.

Further, according to a surface acoustic wave device described in patent document 4, it is proposed that a high sound speed film and low sound speed film are provided between a piezoelectric material substrate and supporting substrate to prevent the leakage of energy of the surface acoustic wave into the supporting substrate. Further, silicon nitride is listed as the high sound speed film and silicon oxide is listed as the low sound speed film. It is, however, not described that the layered part having a relatively high nitrogen concentration as described above is provided in the bonding layer of a material of silicon oxide between the piezoelectric material substrate and supporting substrate to suppress the leakage of the energy of the acoustic wave propagating in the piezoelectric material substrate.

It is further found that it is possible to generate a region having a higher nitrogen concentration in the part of the bonding layer composed of a material of silicon oxide on the side of the piezoelectric material substrate, by providing the silicon oxide film on the supporting substrate, by irradiating nitrogen plasma onto the bonding faces of the silicon oxide film and of the piezoelectric material substrate at 150° C. or lower to activate them and by bonding the bonding face of the piezoelectric material substrate onto the bonding face of the silicon oxide layer.

It is further found that it is possible to generate a region having a higher nitrogen concentration in the intermediate part of the bonding layer composed of a material of silicon oxide, by providing silicon oxide layers on the supporting substrate and piezoelectric material substrate, respectively, by irradiating nitrogen plasma on the respective bonding faces of the respective silicon oxide layers at a temperature of 150° C. or lower to activate them and by directly bonding the silicon oxide film on the supporting substrate and the silicon oxide film on the piezoelectric material substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) shows a piezoelectric material substrate 1, and FIG. 1(b) shows the state that plasma A is irradiated onto a surface of the piezoelectric material substrate 1 to generate an activated surface 5.

FIG. 2(a) shows the state that a silicon oxide film 4 is provided on a supporting substrate 3, and FIG. 2(b) shows the state that plasma B is irradiated onto a bonding face of the silicon oxide film 4.

FIG. 3(a) shows a bonded body 8 of the piezoelectric material substrate 1 and supporting substrate 3, FIG. 3(b) shows the state that the piezoelectric material substrate 1 of a bonded body 8A is thinned by processing, and FIG. 3(c) shows an acoustic wave device 11.

FIG. 4(a) shows the piezoelectric material substrate 1, FIG. 4(b) shows the state that a silicon oxide film 12 is provided on the piezoelectric material substrate 1, and FIG. 4(c) shows the state that plasma A is irradiated onto a bonding face of the silicon oxide film 12 to activate it.

FIG. 5(a) shows a bonded body 18 of the piezoelectric material substrate 1 and supporting substrate 3, FIG. 5(b) shows the state that the piezoelectric material substrate 1 of a bonded body 18A is thinned by processing, and FIG. 5(c) shows an acoustic wave device 21.

FIG. 6 is a graph showing typical resonance characteristics of an acoustic wave device.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The present invention will be described in detail below, appropriately referring to the drawings.

FIGS. 1 to 3 relate to first embodiment of the present invention.

First, as shown in FIG. 1(a), it is prepared a piezoelectric material substrate 1 having a pair of main faces 1 a and 1 b. Then, as shown in FIG. 1(b), plasma is irradiated onto the bonding face 1 a of the piezoelectric material substrate 1 as arrows A to obtain a surface activated bonding face 5.

Further, as shown in FIG. 2(a), it is formed a silicon nitride film 4 on a surface 3 a of a supporting substrate 3. Then, as shown in FIG. 2(b), plasma is irradiated onto a surface 4 a of the silicon nitride layer 4 as arrows B to perform the surface activation to form an activated bonding face 6.

The activated bonding face 5 on the piezoelectric material substrate 1 and activated bonding face 6 of the silicon oxide film 4 on the supporting substrate 3 are contacted and directly bonded with each other to obtain a bonded body 8 shown in FIG. 3(a). Here, it is generated a bonding layer 7 made of a material of silicon oxide, between the supporting substrate 3 and piezoelectric material substrate 1. Provided that the bonding layer 7 is divided into a piezoelectric material substrate-side bonding part 7 a and supporting substrate-side bonding part 7 b, a nitrogen concentration of the piezoelectric material substrate-side bonding part 7 a is made higher than a nitrogen concentration of the supporting substrate-side bonding part 7 b. the piezoelectric material substrate-side bonding part 7 a contacts the piezoelectric material substrate 1, and the supporting substrate-side bonding part 7 b contacts the supporting substrate 3.

An electrode may be provided on the piezoelectric material substrate 1 at this stage. Further, preferably, as shown in FIG. 3(b), the main face 1 b of the piezoelectric material substrate 1 is processed to make the substrate 1 thinner, to obtain a thinned piezoelectric material substrate 1A. 9 represents a processed surface. Then, as shown in FIG. 3(c), an electrode 10 may be formed on the processed surface 9 of the piezoelectric material substrate 1A of the bonded body 8A to obtain a piezoelectric material device 11.

According to the invention of the present embodiment, the bonding layer 7 is made of a material of silicon oxide, and provided that the bonding layer 7 is divided into the piezoelectric material substrate-side bonding part 7 a and supporting substrate-side bonding part 7 b, the nitrogen concentration N7 a of the piezoelectric material substrate-side bonding part 7 a is higher than the nitrogen concentration N7 b of the supporting substrate-side bonding part. On the viewpoint of the invention, a ratio (N7 a/N7 b) of the nitrogen concentration N7 a of the piezoelectric material substrate-side bonding part 7 a with respect to the nitrogen concentration N7 b of the supporting substrate-side bonding part may preferably be 10 or higher and more preferably be 100 or higher. Further, on a practical viewpoint, the ratio (N7 a/N7 b) may preferably be 1000 or lower.

On the viewpoint of the invention, the nitrogen concentration N7 a of the piezoelectric material substrate-side bonding part 7 a may preferably be 1E19 atoms/cm³ or higher and more preferably be 5E19 atoms/cm³ or higher. Further, the nitrogen concentration N7 a may be 8E20 atoms/cm³ or lower. Further, the nitrogen concentration N7 b of the supporting substrate-side bonding part 7 b of the bonding layer may preferably be 4E17 to 7E18 atoms/cm³. Besides, “1E19” indicates

“1×10¹⁹” and the other numerical values are interpreted in the same manner.

Additionally, the bonding layer 7 is divided into the two parts in the direction of thickness to provide the piezoelectric material substrate-side bonding part 7 a and supporting substrate-side bonding part 7 b, provided that the thicknesses of the respective bonding parts are the same. Further, the piezoelectric material substrate-side bonding part 7 a contacts the piezoelectric material substrate 1, and the supporting substrate-side bonding part 7 b contacts the supporting substrate 3.

According to a preferred embodiment, a carbon concentration C7 a of the piezoelectric material substrate-side bonding part 7 a is higher than a carbon concentration C7 b of the supporting substrate-side bonding part 7 b. On the viewpoint, a ratio (C7 a/C7 b) of the carbon concentration C7 a of the piezoelectric material substrate-side bonding part 7 a with respect to the carbon concentration C7 b of the supporting substrate-side bonding part may preferably be 10 or higher and more preferably be 100 or higher. Further, on a practical viewpoint, the ratio (C7 a/C7 b) may preferably be 1000 or lower.

On the viewpoint of the present embodiment, the carbon concentration C7 a of the piezoelectric material substrate-side bonding part 7 a may preferably be 1E19 atoms/cm³ or higher and more preferably be 5E19 atoms/cm³ or higher. Further, the carbon concentration C7 a may be 1E21 atoms/cm³ or lower. Further, the carbon concentration C7 b of the supporting substrate-side bonding pary 7 b of the bonding layer 7 may preferably be 1E17 to 3E18 atoms/cm³.

According to a preferred embodiment, a fluorine concentration F7 a of the piezoelectric material substrate-side bonding part 7 a is higher than a fluorine concentration F7 b of the supporting substrate-side bonding part 7 b. On the viewpoint, a ratio (F7 a/F7 b) of the fluorine concentration F7 a of the piezoelectric material substrate-side bonding part 7 a with respect to the fluorine concentration F7 b of the supporting substrate-side bonding part 7 b may preferably be 5 or higher and more preferably be 50 or higher. Further, on a practical viewpoint, the ratio (F7 a/F7 b) may preferably be 500 or lower.

On the viewpoint of the present embodiment, the fluorine concentration F7 a of the piezoelectric material substrate-side bonding part 7 a may preferably be 3E18 atoms/cm³ or higher and more preferably be 6E18 atoms/cm³ or higher. Further, the fluorine concentration F7 a may be 2E20 atoms/cm³ or lower. Further, the fluorine concentration F7 b of the supporting substrate-side bonding part 7 b of the bonding layer 7 may preferably be 1E17 to 2E18 atoms/cm³.

FIGS. 2, 4 and 5 relate to a second embodiment of the present invention.

First, as shown in FIG. 4(a), it is prepared a piezoelectric material substrate 1 having a pair of main faces 1 a and 1 b. Then, as shown in FIG. 4(b), a silicon oxide film is film-formed on the first main face 1 a of the piezoelectric material substrate 1. Then, as shown in FIG. 4(c), plasma is irradiated onto a bonding face 12 a of the silicon oxide film 12 as arrows A to obtain a surface-activated bonding face 13.

Further, as shown in FIG. 2(a), a silicon oxide film 4 is formed on a surface 3 a of a supporting substrate 3. Then, as shown in FIG. 2(b), plasma is irradiated onto a surface 4 a of the silicon oxide film 4 as arrows B to perform the surface activation, to provide an activated bonding face 6.

Then, an activated bonding face 13 of the silicon oxide layer 12 on the piezoelectric material substrate and the activated bonding face 6 of the silicon oxide film 4 on the supporting substrate are contacted and directly bonded with each other to obtain a bonded body 18 shown in FIG. 5(a). Here, the silicon oxide film 12 and silicon oxide film 4 are integrated through the direct bonding to generate a silicon oxide layer 17.

Provided that the bonding layer 17 is divided into a piezoelectric material substrate-side bonding part 17 a, supporting substrate-side bonding part 17 c and intermediate part 17 b, a nitrogen concentration of the intermediate part 17 b is higher than a nitrogen concentration of the piezoelectric material substrate-side bonding part 17 a and a nitrogen concentration of the supporting substrate-side bonding part 17 c. The piezoelectric material substrate-side bonding part 17 a contacts the piezoelectric material substrate 1, and the supporting substrate-side bonding part 17 c contacts the supporting substrate 3.

An electrode may be provided on the piezoelectric material substrate 1 at this stage. Further, preferably, as shown in FIG. 5(b), the main face 1 b of the piezoelectric material substrate 1 is processed to make the substrate 1 thinner, to obtain a thinned piezoelectric material substrate 1A. 9 represents a processed surface. Then, as shown in FIG. 5(c), a predetermined electrode 10 may be formed on the processed surface 9 of the piezoelectric material substrate 1A of the bonded body 18A to obtain a piezoelectric material device 21.

According to the invention of the present embodiment, the bonding layer 17 is made of a material of silicon oxide, and provided that the bonding layer 17 is divided into the piezoelectric material substrate-side bonding part 17 a, intermediate part 17 b and supporting substrate-side bonding part 17 c, a nitrogen concentration N17 b of the intermediate part 17 b is higher than a nitrogen concentration N17 a of the piezoelectric material substrate-side bonding part 17 a and a nitrogen concentration N17 c of the supporting substrate-side bonding part 17 c. On the viewpoint of the present invention, ratios (N17 b/N17 a) and (N17 b/N17 c) of the nitrogen concentration N17 b of the intermediate part 17 b with respect to the nitrogen concentration N17 a of the piezoelectric material substrate-side bonding part 17 a and the nitrogen concentration N17 c of the supporting substrate-side bonding part 17 c may preferably be 10 or higher and more preferably be 100 or higher. Further, the ratio (N17 b/N17 a) and ratio (N17 b/N17 c) may preferably be 1000 or lower on a practical viewpoint.

Further, the bonding layer 17 is divided, in the direction of thickness, into three parts which are the piezoelectric material substrate-side bonding part 17 a, intermediate part 17 b and supporting substrate-side bonding part 17 c, and the thicknesses of the respective bonding parts 17 a, 17 b and 17 c are made the same. Further, the piezoelectric material substrate-side bonding part 17 a contacts the piezoelectric material substrate 1A, and the supporting substrate-side bonding part 17 c contacts the supporting substrate 3.

On the viewpoint of the present invention, the nitrogen concentration N17 b of the intermediate part 17 b may preferably be 1E19 atoms/cm³ or higher and more preferably be 5E19 atoms/cm³ or higher. Further, the nitrogen concentration N17 b may preferably be 5E20 atoms/cm³ or lower. The nitrogen concentration N17 a of the piezoelectric material substrate-side bonding part 17 a and the nitrogen concentration N17 c of the supporting substrate-side bonding part 17 c may preferably be 9E18 atoms/cm³ or lower. Further, the nitrogen concentrations N17 a and N17 c may preferably be 4E17 atoms/cm³ or higher.

According to the present embodiment, a carbon concentration Cl7 b of the intermediate part 17 b is higher than a carbon concentration Cl7 a of the piezoelectric material substrate-side bonding part 17 a and higher than a carbon concentration Cl7 c of the supporting substrate-side bonding part 17 c. On the viewpoint of the present invention, ratios (C17 b/C17 a) and (C17 b/C17 c) of the carbon concentration C17 b of the intermediate part with respect to the carbon concentration Cl7 a of the piezoelectric material substrate-side bonding part 17 a and the carbon concentration Cl7 c of the of the supporting substrate-side bonding part 17 a may preferably be 10 or higher and more preferably be 100 or higher. Further, the ratio (C17 b/C17 a) and ratio (C17 b/C17 c) may preferably be 1000 or lower on a practical viewpoint.

On the viewpoint of the present invention, the carbon concentration C17 b of the intermediate part 17 b may preferably be 1E19 atoms/cm³ or higher and more preferably be 5E19 atoms/cm³ or higher. Further, the carbon concentration C17 b may preferably be 5E20 atoms/cm³ or lower. The carbon concentration Cl7 a of the piezoelectric material substrate-side bonding part 17 a and the carbon concentration C17 c of the supporting substrate-side bonding part 17 c may preferably be 3E18 atoms/cm³ or lower. Further, the carbon concentrations C17 a and C17 c may be 1E17 atoms/cm³ or higher.

According to the present embodiment, a fluorine concentration F17 b of the intermediate part 17 b is higher than a fluorine concentration F17 a of the piezoelectric material substrate-side bonding part 17 a and higher than a fluorine concentration F17 c of the supporting substrate-side bonding part 17 c. On the viewpoint of the present invention, ratios (F17 b/F17 a) and (F17 b/F17 c) of the fluorine concentration F17 b of the intermediate part 17 b with respect to the fluorine concentration F17 a of the piezoelectric material substrate-side bonding part 17 a and the fluorine concentration Fl7 c of the supporting substrate-side bonding part 17 c may preferably be 5 or higher, and more preferably 50 or higher. Further, the ratio (F17 b/F17 a) and ratio (F17 b/F17 c) may more preferably be 500 or lower on a practical viewpoint.

On the viewpoint of the present invention, the fluorine concentration F17 b of the intermediate part 17 b may preferably be 3E18 atoms/cm³ or higher and more preferably be 6E18 atoms/cm³ or higher. Further, the fluorine concentration F17 b may preferably be 8E19 atoms/cm³ or lower. The fluorine concentration F17 a of the piezoelectric material substrate-side bonding part 17 a and the fluorine concentration Fl7 c of the supporting substrate-side bonding part 17 c may preferably be 2E18 atoms/cm³ or lower. Further, the fluorine concentrations F17 a and F17 c may be 1E17 atoms/cm³ or higher.

The respective constituents of the present invention will be described further in detail below.

Although the material of the supporting substrate 3 is not particularly limited, preferably, the material is selected from the group consisting of silicon, quartz, sialon, mullite, sapphire and translucent alumina. It is thus possible to further improve the temperature characteristics of frequency of an acoustic wave device 11 or 21.

The silicon oxide layers 4 and 12 are formed on the supporting substrate 3 or optionally on the piezoelectric material substrate 1. Although the method of film-forming the silicon oxide layers 4 and 12 is not particularly limited, sputtering, chemical vapor deposition (CVD) and vapor deposition may be listed. Preferably, the supporting substrate 3 is a silicon substrate, and in this case, the silicon oxide film 4 can be formed by sputtering of oxygen or ion injection onto the surface of the silicon substrate, or by heating under oxidizing atmosphere.

The thickness of the silicon oxide film 4 or 12 may preferably be 0.05 μm or larger, more preferably be 0.1 μm or larger and particularly preferably be 0.2 μm or larger, on the viewpoint of the present invention. Further, the thickness of the silicon oxide film may preferably be 3 μm or smaller, preferably 2.5 μm or smaller and more preferably be 2.0 μm or smaller.

The piezoelectric material substrate 1 used in the present invention is made single crystals of lithium tantalate (LT), lithium niobate (LN) or lithium niobate-lithium tantalate solid solution. As the materials have high propagation speeds of a surface acoustic wave and large electro-mechanical coupling factors, it is preferred for use in a surface acoustic wave device for high frequency and wide-band frequency applications.

Further, the normal direction of the main surface of the piezoelectric single crystal substrate 1 is not limited. For example, in the case that the piezoelectric material substrate 1 is made of LT, it is preferred to use the substrate rotated from Y-axis toward Z-axis by 32 to 50° (180°, 58° to 35°, 180° on Eulerian angle representation) around X-axis, which is a direction of propagation of a surface acoustic wave, because of a low propagation loss. In the case that the piezoelectric material substrate 1 is made of LN, (i) it is preferred to use the substrate rotated from Z-axis toward −Y-axis by 37.8° (0°, 37.8°, 0° on Eulerian angle representation) around X-axis, which is a direction of propagation of a surface acoustic wave, because of a large electro-mechanical coupling factor. Alternatively, (ii) it is preferred to use the substrate rotated from Y-axis toward Z-axis by 40 to 65° (180°, 50 to 25°, 180° on Eulerian angle representation) around X-axis, which is a direction of propagation of a surface acoustic wave, because a high acoustic speed can be obtained. Further, although the size of the piezoelectric material substrate 1 is not particularly limited, for example, the diameter may be 100 to 200 mm and thickness may be 0.15 to 1 μm.

Plasma is then irradiated onto the bonding face 1 a of the piezoelectric material substrate 1, the bonding face 12 a of the silicon oxide film 12 on the piezoelectric material substrate 1 or bonding face 4 a of the silicon oxide film 4 on the supporting substrate 3 at a temperature of 150° C. or lower to activate the bonding face 1 a, 4 a or 12 a. Although it is preferred to irradiate nitrogen plasma on the viewpoint of the present invention, the bonded bodies 8A and 18A of the present invention can be obtained even in the case that oxygen plasma is irradiated.

The pressure during the surface activation may preferably be 100 Pa or lower and more preferably be 80 Pa or lower. Further, the atmosphere may be composed of nitrogen only, oxygen only, or mixture of nitrogen and oxygen.

The temperature during the irradiation of the plasma is made 150° C. or lower. It is thereby possible to obtain the bonded bodies 8 and 18A, each having a high bonding strength and no deterioration of the crystallinity. On the viewpoint, the temperature during the plasma irradiation is made 150° C. or lower and may more preferably be 100° C. or lower.

Further, the energy of the irradiated plasma may preferably be 30 to 150 W. Further, a product of the energy of the irradiated plasma and irradiation time period may preferably be 0.12 to 1.0 Wh.

The bonding face 5 and bonding face 6 of the substrates after the plasma treatment or the bonding face 13 a on bonding face 6 are contacted with each other at a room temperature. Although these may be treated under vacuum, the bonding faces may preferably be contacted under atmosphere. It is thus possible to realize a composite substrate maintaining the distribution of nitrogen, carbon and fluorine incorporated into silicon oxide by plasma treatment.

According to a preferred embodiment, the bonding face 1 a of the piezoelectric material substrate 1 and the bonding faces 4 a and 12 a of the silicon oxide layers 4 and 12 are subjected to flattening process before the plasma treatment. The method of flattening the respective bonding faces 1 a,4 a and 12 a includes lapping, chemical mechanical polishing (CMP) and the like. Further, the flattened face may preferably have Ra of lnm or lower and more preferably have Ra of 0.3 nm or lower.

The bonding face 5 of the piezoelectric material substrate 1 and the bonding face 6 of the silicon oxide film 4 on the supporting substrate 3, or the bonding face 13 of the silicon oxide film 12 on the piezoelectric material substrate 1 and bonding face 6 of the silicon oxide film 4 on the supporting substrate 3 are contacted and bonded with each other. Thereafter, annealing treatment may preferably be performed to improve the bonding strength. The temperature during the annealing temperature may preferably be 100° C. or higher and 300° C. or lower.

The bonded bodies 8A and 18A of the present invention may preferably be applied as acoustic wave devices 11 and 21.

As acoustic wave devices 11 and 21, a surface acoustic wave device, Lamb wave-type device, thin film resonator (FBAR) or the like is known. For example, the surface acoustic wave device is produced by providing an input side IDT (Interdigital transducer) electrodes (also referred to as comb electrodes or interdigitated electrodes) for oscillating surface acoustic wave and IDT electrode on the output side for receiving the surface acoustic wave on the surface of the piezoelectric material substrate. By applying high frequency signal on the IDT electrode on the input side, electric field is generated between the electrodes, so that the surface acoustic wave is oscillated and propagated on the piezoelectric material substrate. Then, the propagated surface acoustic wave is drawn as an electrical signal from the IDT electrodes on the output side provided in the direction of the propagation.

A material forming the electrode 10 of the piezoelectric material substrate 1A may preferably be aluminum, an aluminum alloy, copper or gold, and more preferably be aluminum or the aluminum alloy. The aluminum alloy may preferably be Al with 0.3 to 5 weight % of Cu mixed therein. In this case, Ti, Mg, Ni, Mo or Ta may be used instead of Cu.

EXAMPLES Inventive Example A1

It was produced an acoustic wave device 11 shown in FIG. 3(c), according to the method described referring to FIGS. 1 to 3.

Specifically, it was prepared a 42Y-cut X-propagation LiTaO₃ substrate (piezoelectric material substrate) 1 having a thickness of 200 μm and both main faces polished into mirror surfaces and a high-resistance (>2 kΩ·cm) Si (100) substrate (supporting substrate) 3 having a thickness of 675 μm. Both substrates have sizes of 150 mm, respectively. A silicon oxide layer 4 was film-formed by sputtering in a thickness of 500 nm on the supporting substrate 4. As the silicon oxide layer 4 after the film-formation had a surface roughness Ra of 0.6 nm, the surface was slightly polished by CMP (Chemical mechanical polishing) so that Ra was improved to 0.3 nm.

The bonding face 1 a of the piezoelectric material substrate 1 and the bonding face 4 a of the silicon oxide layer on the supporting substrate 3 were subjected to cleaning and surface activation, respectively. Specifically, ultrasonic cleaning using pure water was performed, and the substrate surfaces were dried by spin dry. The supporting substrate 3 after the cleaning was then introduced into a plasma activation chamber, and the bonding face 4 a was activated by nitrogen gas plasma at 30° C. Further, the piezoelectric material substrate 1 was similarly introduced into the plasma activation chamber, and the bonding face 1 a was subjected to surface activation by nitrogen gas plasma at 30° C. The time period of the surface activation was made 40 seconds and the energy was made 100 W. The ultrasonic cleaning and spin dry as described above were performed again for removing particles adhered during the surface activation.

Then, the positioning of the respective substrates were performed, and the bonding faces 5 and 6 of the substrates were contacted with each other at room temperature. The substrates were contacted with the piezoelectric material substrate 1 positioned upper side. As a result, it was observed the state (so called-bonding wave) that the adhesion of the substrates was spreading, indicating that good preliminary bonding was completed. Then, the bonded body was charged into an oven filled with nitrogen atmosphere and held at 120° C. for 10 hours, for improving the bonding strength.

The surface 1 b of the piezoelectric material substrate 1 of the bonded body 8 after the heating was subjected to grinding, lapping and CMP polishing so that the thickness of the piezoelectric material substrate 1A was made 16, 8, 4, 2 or 1 μm. The nitrogen concentrations, carbon concentrations and fluorine concentrations at the respective parts were measured using SIMS method (secondary ion mass spectroscopy) for the thus obtained bonded bodies 8A. As the SIMS measuring system, “CAMECA IMS-7f” was used, Cs+ was used as primary ion specie, and the primary acceleration voltage was made 15.0 kV. The detection region was made 30 μmφ.

The thickness of the piezoelectric material substrate 1A was measured by an optical measurement system (“F20” supplied by Filmetrix Corporation) applying optical interference. An IDT electrode 10 made of aluminum metal was provided on the surface 9 of the polished piezoelectric material substrate 1A by photolithography process. The period λ of the electrode was made 4 μm, so that oscillation frequency becomes about 1000 MHz. The thickness of the IDT electrode 10 was made 200 nm and it was provided 80 pairs of reflectors on both sides of the 200 pairs of IDT electrodes 8, respectively, to produce an acoustic wave device 11 (SAW resonator) of 1 port. The impedance characteristics of the thus produced acoustic wave device (SAW resonator) 11 was measured by a network analyzer “E5071C” supplied by Agilent Corporation. FIG. 6 shows typical resonance characteristics. It was thus observed a resonance peak at about 1000 MHz (fs) and anti-resonance peak at about 1050 MHz (fr). The ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was measured. The results were shown in table 3.

Inventive Example A2

In the inventive example A1, plasma of mixed gases of 80 percent of nitrogen gas and 20 percent of oxygen gas was used instead of nitrogen plasma. When the gas composition was changed, the matching was appropriately changed so that the reflection electric power took the minimum value. The other conditions were the same as those in the inventive example A1.

According to same procedure as the inventive example A1, nitrogen concentrations, carbon concentrations and fluorine concentrations at the respective parts in the thus obtained bonded body 8 were measured and the measurement results were shown in table 1. Further, as the inventive example A1, the piezoelectric material substrate was processed so that the thickness of the piezoelectric material substrate 1A was made 16, 8, 4, 2 or 1 μm. The ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was measured. The results were shown in table 3.

Comparative Example A1

According to the inventive example A1, each bonded body and SAW device were produced and subjected to the same measurement as the inventive example A1. However, the silicon oxide film was film-formed on the piezoelectric material substrate 1 and not film-formed on the side of the supporting substrate 3. Further, the surface activation of the silicon oxide film and supporting substrate 3 was performed, and the activated surfaces of the silicon oxide film and supporting substrate 3 were directly bonded by irradiating nitrogen plasma.

The nitrogen concentrations, carbon concentrations and fluorine concentrations at the respective parts were measured for the thus obtained bonded bodies, and the measurement results were shown in table 2. The piezoelectric material substrate 1 was processed so that the thickness of the piezoelectric material substrate was made 16, 8, 4, 2 or 1 μm according to the procedure as the inventive example A1, and the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fs was measured. The results were shown in table 3.

TABLE 1 Nitrogen Carbon Fluorine concen- concen- concen- tration tration tration Measurement points atoms/cm³ atoms/cm³ atoms/cm³ Inventive Average value of 7.1E19 2.6E20 4.4E19 Example piezoelectric A1 material substrate- side bonding part Average value of 4.6E18 3.2E17 9.7E17 supporting substrate side bonding part Inventive Average value of 3.2E19 2.1E20 9.9E19 Example piezoelectric A2 material suhstrate- side bonding part Average value of 1.8E18 1.6E17 3.4E17 supporting substrate- side bonding part

TABLE 2 Nitrogen Carbon Fluorine concen- concen- concen- tration tration tration Measurement points atoms/cm³ atoms/cm³ atoms/cm³ Comparative Average value of 1.6E18 8.1E17 2.3E17 Example piezoelectric A1 material substrate- side bonding part Average value of 8.3E19 1.2E20 8.8E18 supporting substrate- side bonding part

TABLE 3 Ratio of admittance value (dB) Thickness of piezoelectric material substrate after Inventive Inventive Comparative processing (μm) Example A1 Example A2 Example A1 16.0 42.5 44.0 43.3 8.0 40.7 42.5 42.8 4.0 43.3 42.2 40.0 2.0 44.3 45.1 39.3 1.0 46.5 44.9 35.8

As shown in table 3, even in the case that the piezoelectric material substrate 1A is extremely thinned by processing to a thickness of 2.0 to 1.0 μm in an acoustic wave device (SAW device) 11 using the bonded body 8A of the present invention, the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was not deteriorated and was good. On the other hand, according to the bonded body of the comparative example A1, as the thickness of the piezoelectric material substrate is smaller, the ratio of the admittance valuer at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was deteriorated. In particular, according to the bonded body of the comparative example A1, as the piezoelectric material substrate was processed to a thickness of 4.0 μm or smaller, the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was considerably deteriorated.

Inventive Example B

In the inventive example A1, the material of the piezoelectric material substrate was changed to a 128Y-cut X-propagation lithium niobate. As a result, it was obtained the same results as those of the inventive example A1.

Inventive Example C1

It was produced an acoustic wave device 21 shown in FIG. 5(c), according to the method illustrated referring to FIGS. 2, 4 and 5.

Specifically, it was prepared a 42Y-cut X-propagation LiTaO₃ substrate (piezoelectric material substrate) 1 having a thickness of 200 μm and both main faces polished into mirror surfaces and a high-resistance (>2 kΩ·cm) Si (100) substrate (supporting substrate) 3 having a thickness of 675 μm. Both substrates have sizes of 150 mm, respectively. Silicon oxide films 12 and 4 were film-formed by sputtering in thicknesses of 250 nm on the piezoelectric material substrate 1 and supporting substrate 3, respectively. As the respective silicon oxide films 12 and 4 after the film-formation each had a surface roughness Ra of 0.6 nm measured by AFM, the surface was slightly polished by CMP (Chemical mechanical polishing) so that Ra was improved to 0.3 nm.

The bonding face 12 a of the silicon oxide film 12 on the piezoelectric material substrate 1 and the bonding face 4 a of the silicon oxide layer 4 on the supporting substrate 3 were then subjected to washing and surface activation, respectively. Specifically, ultrasonic cleaning using pure water was performed and the substrate surface were dried by spin drying.

The supporting substrate 3 after the cleaning was then introduced into a plasma activation chamber to activate the bonding face 4 a of the silicon oxide film 4 with nitrogen gas plasma at 30° C. Further, the piezoelectric material substrate 1 was similarly introduced into the plasma activation chamber so that the bonding face 12 a of the silicon oxide film 12 was subjected to surface activation with nitrogen gas plasma at 30° C. The time period of the surface activation was made 40 seconds and energy was made 100 W. The ultrasonic cleaning and spin dry as described above were performed again for removing particles adhered during the surface activation.

Then, the positioning of the respective substrates was performed, and the activated bonding faces 13 and 6 of the substrates were contacted with each other at room temperature. The substrates were contacted with the piezoelectric material substrate 1 positioned upper side. As a result, it was observed the state (so called-bonding wave) that the adhesion of the substrates was spreading, indicating that good preliminary bonding was completed. Then, the bonded body was charged into an oven filled with nitrogen atmosphere and held at 120° C. for 10 hours, for improving the bonding strength.

The surface of the piezoelectric material substrate 1 of the bonded body 18 after the heating was subjected to grinding, lapping and CMP processing so that the thickness of the piezoelectric material substrate 1A was made 16, 8, 4, 2 or 1 μm. The nitrogen concentrations, carbon concentrations and fluorine concentrations at the respective parts were measured using SIMS method (secondary ion mass spectroscopy) for the thus obtained bonded bodies 18A. As the SIMS measuring system, “CAMECA IMS-7f” was used, Cs+ was used as primary ion specie, and the primary acceleration voltage was made 15.0 kV. The detection region was made 30 μmφ. The results were shown in table 4.

An IDT electrode 10 made of aluminum metal was provided on the processed surface 9 of the polished piezoelectric material substrate 1A, and an acoustic wave device 21 (SAW device) of 1 port by photography process. The period λ of the electrode was made 4 μm, so that the oscillation frequency was about 1000 MHz. The impedance characteristics of the thus produced acoustic wave device (SAW resonator) 21 was measured by a network analyzer “E5071C” supplied by Agilent corporation. It was measured the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr. The results were shown in table 6.

Inventive Example C2

In the inventive example C1, it was used plasma of mixed gases of 80 percent of nitrogen gas and 20 percent of oxygen gas, instead of nitrogen plasma. The experiment was performed according to the same procedure as that in the inventive example C1 except that.

The nitrogen concentrations, carbon concentrations and fluorine concentrations at the respective parts were measured for the thus obtained bonded body 18A according to the same procedure as the inventive example C1, and the measurement results were shown in table 4. Further, the piezoelectric material substrate 1A was processed to thicknesses of 16, 8, 4, 2 and 1 μm according to the same procedure as that of the inventive example C1, and the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fs was measured. The results were shown in table 6.

Comparative Example C1

The respective bonded bodies and acoustic wave devices (SAW devices) were produced according the same procedure as the inventive example C1 and subjected to the measurements same as those of the inventive example C1. However, the silicon oxide film was film-formed on the piezoelectric material substrate and not film-formed on the side of the supporting substrate. Further, the silicon oxide film and supporting substrate were subjected to the surface activation, and the activated surfaces of the silicon oxide film and supporting substrate were directly bonded with each other by irradiation of nitrogen gas plasma.

The nitrogen concentration, carbon concentration and fluorine concentration at the respective parts were measured for the thus obtained bonded body and the measurement results were shown in table 5. Further, the piezoelectric material substrate 1A was processed to thicknesses of 16, 8, 4, 2, and 1 μm as the same procedure as that the inventive example A1, and the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fs was measured. The results were shown in table 6.

TABLE 4 Nitrogen Carbon Fluorine concen- concen- concen- tration tration tration Measurement points atoms/cm³ atoms/cm atoms/cm³ Inventive Average value of 1.8E18 5.3E17 6.3E17 Example piezoelectric C1 material substrate- side bonding part Average value of 6.1E19 9.9E19 3.2E19 intermediate part Average value of 2.6E18 4.1E17 9.7E17 supporting substrate- side bonding part Inventive Average value of 8.8E17 4.9E17 2.8E17 Example piezoelectric C2 material substrate- side bonding part Average value of 2.9E19 1.7E20 9.5E18 intermediate part Average value of 7.6E17 2.5E17 4.4E17 supporting substrate- side bonding part

TABLE 5 Nitrogen Carbon Fluorine concen- concen- concen- tration tration tration Measurement points atoms/cm² atoms/cm² atoms/cm² Comparative Average value of 1.6E18 8.1E17 4.3E17 Example piezoelectric C1 material substrate- side bonding part Average value of 1.6E18 8.1E17 6.6E17 intermediate part Average value of 8.3E19 1.2E20 1.2E19 supporting substrate- side bonding part

TABLE 6 Ratio of admittance values (dB) Piezoelectric material substrate Inventive Inventive Comparative after processing Example C1 Example C2 Example C1 16.0 43.9 41.4 43.3 8.0 42.2 42.5 42.8 4.0 44.8 42.1 40.0 2.0 46.6 43.3 39.3 1.0 47.3 46.1 35.8

As shown in table 6, according to the acoustic wave device (SAW device) 21 using the bonded body 18A of the present invention, even in the case that the thickness of the piezoelectric material substrate 1A was made extremely small as 2.0 to 1.0 μm by processing, the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was not deteriorated and was good. On the other hand, according to the bonded body of the comparative example C1, as the thickness of the piezoelectric material substrate was thinner, the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fs was deteriorated. In particular, according to the bonded body of the comparative example C1, as the thickness of the piezoelectric material substrate was made 4.0 μm or smaller by processing, the ratio of the admittance value at the resonance frequency fs with respect to the admittance value at the anti-resonance frequency fr was considerably deteriorated.

Inventive Example D

In the inventive example C1, the material of the piezoelectric material substrate 1 was changed to lithium niobate. As a result, it was obtained similar results as those of the inventive example C1. 

1. A bonded body comprising: a supporting substrate; a piezoelectric material substrate comprising a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate; and a bonding layer for bonding said supporting substrate and said piezoelectric material substrate, wherein said bonding layer comprises a material of silicon oxide; and wherein a nitrogen concentration of an intermediate part is higher than a nitrogen concentration of a piezoelectric material substrate-side bonding part and a nitrogen concentration of a supporting substrate-side bonding part, provided that said bonding layer is divided into said piezoelectric material substrate-side bonding part, said supporting substrate-side bonding part and said intermediate part between said piezoelectric material substrate-side bonding part and said supporting substrate-side bonding part, and wherein a carbon concentration of said intermediate part is higher than a carbon concentration of said piezoelectric material substrate-side bonding part and a carbon concentration of said supporting-substrate side bonding part.
 2. The bonded body of claim 1, wherein a fluorine concentration of said intermediate part is higher than a fluorine concentration of said piezoelectric material substrate-side bonding part and a fluorine concentration of said supporting substrate-side bonding part.
 3. The bonded body of claim 1 or 2, wherein said piezoelectric material substrate has a thickness of 4.0 μm or smaller.
 4. A bonded body comprising: a supporting substrate; a piezoelectric material substrate comprising a material selected from the group consisting of lithium niobate, lithium tantalate and lithium niobate-lithium tantalate; and a bonding layer for bonding said supporting substrate and said piezoelectric material substrate, wherein said bonding layer comprises a material of silicon oxide; and wherein a nitrogen concentration of an intermediate part is higher than a nitrogen concentration of a piezoelectric material substrate-side bonding part and a nitrogen concentration of a supporting substrate-side bonding part, provided that said bonding layer is divided into said piezoelectric material substrate-side bonding part, said supporting substrate-side bonding part and said intermediate part between said piezoelectric material substrate-side bonding part and said supporting substrate-side bonding part, and wherein a fluorine concentration of said intermediate part is higher than a fluorine concentration of said piezoelectric material substrate-side bonding part and a fluorine concentration of said supporting substrate-side bonding part.
 5. The bonded body of claim 4, wherein said piezoelectric material substrate has a thickness of 4.01 μm or smaller. 